Nolan (1935) named the Gerster for
Gerster Gulch in the western Deep Creek Range, Utah. As originally defined in T.
7 S., R. 19 W., the Gerster contained rocks now mapped as the Kaibab Formation
(Steele, 1960).

Geologic Age

The Gerster is Permian
(Guadalupian) in age as determined from brachiopod fauna. At its type locality,
it overlays the upper portion of the Oquirrh Formation also known as the Loray
Formation of Steele (1960). The Gerster forms the upper unit of the Park City
Group throughout its regional extent (Wardlaw and others, 1979).

General Lithology

The Gerster Formation is generally
composed of light-brown-gray, resistant, coarse to very coarse-grained
limestone, and interbedded thicker units of less resistant, yellowish-gray,
argillaceous limestone, containing finely laminated lime mud and terriginous
quartz grains (Hose and Blake, 1976). The ledge forming limestones often contain
abundant fragments of brachiopods, mollusks, crinoids, bryozoans and echinoid
spines. Scattered dark rusty brown weathering chert nodules are present
throughout the unit. As the Gerster is traced to the north and northeast into
the Leach Mountains and across the Utah state line, it appears to grade
laterally into the shales and cherts of the Phosphoria Formation (Steele, 1960;
Bissell, 1964).

In the Butte Mountains, the Gerster
is over 1,600 feet of gray, pink, red, and brown interbedded skeletal, micritic,
and skeletal-detrital limestones which are commonly encrinal and contain
abundant bryozoans, brachiopods, and sponges (Douglass, 1960; Bissell, 1964).
Sides (1966) also noted possible algae within the silty and sandy limestone of
the Gerster, and reported thin-bedded, light-gray chert about 200 feet below the
top of the formation and several thin chert-pebble conglomerate beds in the
middle and upper part of the formation.

In the southern Pequop Mountains,
the Gerster Formation is composed of light gray, thin to thick-bedded limestone
with gray and brown, nodular and banded chert throughout the unit, and a few
thin beds of fine-grained chert-granule conglomerate (Yochelson and Frazer,
1973). Between Indian and Burnt Creeks in the southern Pequop Mountains, Bissell
(1964) divided the Gerster into an upper and lower portion. The lower portion is
composed of about 190 feet of thin to thick-bedded and massive, red to
yellow-gray, skeletal limestone, sandy micritic limestone, and slightly cherty
limestone. The upper 225 feet are laminated and thin to medium-bedded,
yellow-gray to orange-brown, sandy limestone, encrinal and brachiopod-rich
skeletal limestone, and argillaceous bryozoan-bearing limestone (Bissell, 1964).

In the northern Cherry Creek Range
and at Phalen Butte north of Currie the Gerster is composed of over 500 feet of
thin to thick-bedded and massive, cherty and sandy limestone, brachiopod,
bryozoan and crinoid-rich limestone and minor calcareous sandstone (Bissell,
1964).

In the Medicine Range, Collinson
(1968) described upper and lower portions of the Gerster. The upper portion is
massive, light-gray skeletal limestone, interbedded with less resistant
thin-bedded, sandy limestone and siltstone, and brown weathering chert nodules
which form 10 to 60 percent of the unit. The lower portion of the Gerster is
slope forming calcareous siltstone and interbedded dolomitic limestone. In the
Medicine Range the Gerster lies conformably on the Plympton Formation and is
unconformably overlain by the Triassic Thaynes Formation. Bissell (1964)
reported brachiopods, bryozoans, and crinoids in the skeletal-detrital
limestones in the Medicine Range.

The Gerster Formation in the Leach
Mountains is composed of an upper portion of alternating gray, silty, medium to
thick-bedded, fossiliferous limestone underlain by lenses and nodules of brown
chert beds which compose as much as 50 percent of some sections (Martindale,
1981). The fossils in these limestones include brachiopods, crinoids, bryozoans,
algae, and scaphopods (Martindale, 1981). Dark gray-brown dolomitic and
phosphatic mudstones with horizontal algal laminations are present in the middle
portion of the unit.

Average Thickness

Although Bissell (1964) reported a
thickness of 4,190 feet for the Gerster in the Medicine Range, the section is
repeated several times along faults and is only about 1,200 feet thick according
to Collinson (1968). It is 140 to 376 feet thick in the southern Pequop
Mountains according to Yochelson and Frazer (1973), and Bissell (1964) measured
about 415 feet in the southern Pequop Range in the Burnt Creek-Indian Creek
area. It is about 1,638 feet in the central Butte Mountains (Sides, 1966), 1,685
feet in the Leach Range (Martindale, 1981), 585 feet in the Cherry Creek Range
(T. 27 N., R. 63 E., Bissell, 1964), and about 742 feet at Phalen Butte
northwest of Currie (Bissell, 1964).

Areal Distribution

The Gerster has been found in the
Snake (formerly Burnt Canyon), Pequop, northern Butte, and Ferguson Springs
Mountains, northern Cherry Creek, Maverick Springs, and Medicine Ranges,
northwest of Currie at Phalen Butte, just north of the Kern Mountains, northern
Schell Creek and Antelope Ranges, and Leach Mountains.